1. Technical Field
Embodiments of the present invention are directed to a wafer-level package for an optical sensor device, and in particular, to a package using flip-chip solder bumps to couple the optical sensor to a fan-out redistribution layer on an optically transparent substrate.
2. Description of the Related Art
An optical sensor is the central and fundamental element of a digital camera. The optical sensor captures an image and converts it to electronic data that is transmitted to other circuits of the camera for processing, display, storage, transmission to other devices, etc.
FIG. 1 shows a side cross-sectional view of a portion of a prior art optical sensor package 100. The optical sensor package 100 includes a semiconductor material substrate 102 on which is formed the optical sensor itself. A glass cover 106 is coupled to an upper surface of the semiconductor substrate 102 by an adhesive layer 108, with a small air cavity 110 between the glass layer and the optical sensor 104. Devices that are formed on semiconductor material substrates are generally formed on only one surface thereof, and actually occupy a very small part of the total thickness of the substrate. Typically, after the formation of an electronic device on a substrate, additional passivating and insulating layers are deposited over the device, and contact terminals of the device are formed over the insulating layers. In the case of the optical sensor device of FIG. 1, because the optical sensor 104 must remain exposed to receive optical images, contact terminals 112 are formed around the perimeter of the optical sensor 104. In order to access the image data from the optical sensor 104, a camera or other device must have a connection to the contact terminals 112.
A challenge that designers face with respect to coupling the optical sensor 104 to a camera circuit is that the device must be positioned with the optical sensor facing outward, and without any obstructions that would interfere with the reception of an optical image. This makes it difficult to couple a circuit to contact pads 112 that are on a side facing away from the printed circuit board, without the benefit of more traditional packaging techniques, which are not compatible with the correct operation of the optical sensor.
FIG. 1 shows one of a plurality of through-silicon-vias (TSV) 114 that is formed in the substrate 102, and by which electrical contact is made with contact pads 112 on the opposite face of the substrate 102. Each TSV includes an aperture 116 that is etched from the backside of the substrate 102 and extends to a respective contact pad 112 on the front. Following the formation of the apertures 116, a conductive layer 120 is deposited on the back surface of the substrate 102 and inside the aperture 116, forming an electrical contact between the conductive layer 120 and the contact pad 112. The conductive layer 120 is then etched to form electrical traces 122 extending from the contact pads 112 to the back surface of the substrate 102. A passivation layer 124 is then formed over the back surface of the substrate 102, covering the electrical traces 122. The passivation layer 124 is masked and etched to form an aperture 126, which exposes one end of the electrical trace 122. Another conductive layer is deposited and etched to form a contact pad 128 over the passivation layer 124, which is in electrical contact with the electrical trace 122. Finally, a solder ball is formed on the contact pad 128.
The TSV structure, including the solder ball 130, is one of a very large plurality of such structures that are distributed around the perimeter of the optical sensor 104. To couple the optical sensor package 100 to an electronic circuit, the package 100 is positioned on a printed circuit board, with each of the solder balls 130 in physical contact with a corresponding contact pad of the circuit board. The package 100 and circuit board are then heated to a temperature sufficient to melt the solder balls, which reflow and form mechanical and electrical connections between the optical sensor package 100 and the electrical circuit of the printed circuit board.
FIG. 2 shows a simplified diagrammatic plan view of the optical sensor package 100, showing the position of the optical sensor 104 in relation to the TSV structures 114, the electrical traces 122 and the solder balls 130, although the arrangement shown is merely exemplary. In practice, the distribution of the TSVs 114 and the solder balls 130 may be less regular, and there may be many more than are shown here.
There are a number of problems associated with optical sensor packages of the type described with reference to FIGS. 1 and 2. First, the use of TSV structures around the perimeter of the optical sensor 104 can increase the necessary size of the semiconductor substrate, relative to the dimensions of the optical sensor, because a TSV requires more space than would be necessary for a conventional contact pad, for example. This reduces the number of chips that can be formed on a semiconductor wafer, which in turn increases the cost, per unit. Second, the apertures of the TSV structures can potentially weaken the substrate. In the case where a large number of apertures are positioned in a row, the substrate is more susceptible to cracking along that line of apertures. Third, the locations of the solder balls 112 directly opposite the optical sensor 104 create points where the device is rigidly supported from below, while it is largely unsupported elsewhere. This can present problems even before the device is sold to a customer. After a device is manufactured and before it is shipped, the device undergoes testing, during which it is placed in a fixture with contact points in positions corresponding to the solder balls 112. Unless it is handled with extreme care during the testing procedure, the device can be damaged. For example, if excessive or uneven pressure is applied to the upper surface of the glass cover as it is being placed in the fixture, the concentration of forces at the solder balls can cause fractures in electrical traces or dielectric layers, which can result in immediate failure of the device, during the final parametric tests, or can produce a delayed failure, which may occur after the device has been integrated into a camera and sold. Additionally, pressure on the upper surface of the glass cover 106, which is transferred to the substrate 102 around the perimeter of the device, can cause the outer portion of the substrate 102 to flex downward, and the central portion of the substrate to flex upward, resulting in bending of the membrane of the optical sensor, and a loss of parallelism between the surface of the optical sensor and the inner surface of the glass cover in the air cavity 110 (these surfaces should be precisely parallel). This results in interference lines appearing in images captured by the sensor 104. This can also occur if a bracket or piece of trim in the camera applies pressure to the edge of the device. Finally, some optical sensor devices have shown a tendency to transmit light from behind the device, via the TSV structure, which can degrade the quality of a captured image.